It is known that a system of positioning a transport aeroplane (on the ground and in flight) is generally based on GPS/GNSS signals relating to positioning satellites. The synchronisation of signals is obtained by atomic clocks on board each satellite. The receiver installed on board the aeroplane compares the offset of the signal received in relation to the signal generated locally in the receiver and thus measures the distance from the satellite. These measurements are repeated on all the visible satellites (that is to say on all the satellites for which a signal is received on the aeroplane) and make it possible to calculate a position continuously.
Any measurement of distance, regardless of the system used (low earth orbit or geostationary satellite constellation or local beacon), places the receiver (mounted on the aeroplane) on a sphere centred on the transmitter. Using at least three transmitters, these spheres have a single point of intersection. However, this simple principle is complicated. This is because: the local clock of the receiver rarely has atomic precision. Therefore only the time differences are precise, which requires four beacons or satellites in order to define a point instead of three (if the altitude is known, three beacons are sufficient); the receiver is movable, and the measurements are therefore performed at different points; and the radio waves for transmission of the signals have a speed which is slightly variable depending upon the ionospheric layers through which they pass.
The position signal receiver incorporates these various errors, using corrections and measurements from various satellites or beacons, and integration and filtering techniques such as Kalman filters, in order to obtain the most probable point and its estimated precision and speed as well as the universal time.
For precision applications or procedures, such as a procedure of the RNP (Required Navigation Performance) type for example, requiring absolute security of navigation of the aircraft (navigation procedures in restricted corridors up to 0.1 NM), the navigation signals are supplemented by a so-called “integrity” signal which makes it possible to eliminate any measurement coming from a transmitter which is defective temporarily or for a prolonged period. The integrity is a measure of the confidence which the user can have in the quality of the outputs from the system (that is to say the information supplied by the system).
On board aeroplanes, calculation means are generally used for calculating the position based on hybridisation between GPS/GNSS data and inertial data of the IRS (Inertial Reference System) type. GPIRS (Global Positioning/Inertial Reference System) hybridisation consists of damping or stabilising the divergent errors of an inertial navigation unit by means of a positioning measurement derived from GPS/GNSS data.
It is known that, in the context of air transport, before performing air operations in the take-off, approach, landing phase etc., in accordance with the regulations in force, in particular operations of the RNP type, the airline companies are requested to proceed with a prediction on the ground of the availability of the hybrid position and the associated integrity, in the zone and at the time planned for landing the aircraft.
In the conventional manner, an algorithm of the GPIRS Kalman filter of an inertial unit is modelled in ground prediction software. This ground prediction software then makes it possible to make a prediction, before the departure of the aircraft, of the precision, the availability and the integrity of the aeroplane position calculated by the GPIRS Kalman filter, at the given point(s) and the given instant(s).
However, the conventional tools for prediction of the precision, the integrity and the availability of the aeroplane position (GPS/GNSS or GPIRS) have different limitations, in particular no ground prediction tool covers the monitoring of the onboard hardware integrity in the calculation of the total integrity of the position.
In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.